In order to machine wood blanks according to the above process, planing machines, automatic four-edge planing machines, automatic thicknessing machines, automatic profile cutting machines, planing, grooving and keying machines and others are used.
The rotating cutting tools work on the wood blank against its advance direction, so that during the advance the cutting reactions translate into a resistance which has to be overcome. When the front end of the wood blank has passed the cutting tool and has made contact with the first advance roller, then the latter is capable to insure the further advance with its own conveying force, namely with the optimal advance speed required by the machining process.
The known processes have certain problems related to the conveying of the wood blank into the machine, past the rotating tool up to the point of contact with the first advance roller.
Manual feeding is widely used. This procedure requires a certain practice and experience, as well as persistence and concentration of the operator, in order to feed the blanks with the necessary force and at the same time with the required constant advance speed until the front end of the blank reaches the first advance roller. Even experienced operators make mistakes at times, with the result that during the machining process or as the rotating tool is passed, the front end of the wood blank receives chatter marks as a result of the uncontrolled advance speed and the variable contact pressure.
Manual feeding is labor intensive and therefore expensive. At today's state of the art it is acceptable only in a craftsmanship operation. At least the front ends of the wood blanks damaged by chatter marks have to be cut off, since they are unsuitable for further use. So there is more or less waste. However, if the wood blanks have been precut to the desired length, then the finished wood pieces with chatter marks are completely wasted.
As a rule, processing machines for wood blanks also have input rollers, which insure the required advance speed and the constant contact pressure. They are arranged immediately upstream of the rotating tool, opposite the support and guide surface.
In practice the use of input rollers is avoided whenever possible. The rollers are frequently pulled off their shafts, thus being removed. Usually they are arranged at the greatest possible distance from the support and guide surface and secured so that they do not come in contact with the wood blanks under any circumstances. Such operations can only be performed when the processing machine is at a standstill.
The reason for this procedure is that the wood blanks which have been processed with input rollers are usually fraught with irregularities, e.g. they particularly lack the rectilinear precision required for profiles for windows, furniture and so on.
The cause of this lack of rectilinear precision is the relatively high contact pressure of the input roller. However it is unavoidable, since the first cut or the first contact between the rotating cutting tool and the frontal surface of the front end of the wood blank sometimes causes shock or impact-like cutting reactions which, without a strong hold on the blank by corresponding contact pressure of the input roller, causes the blank to be pushed back or ejected.
However this relatively high contact pressure of the input roller, indispensable for the onset of cutting, has proven to be detrimental over the length of the blank. Each approximately linear compression load application on a wood blank resting on a rigid support causes a bending effect, under the influence of which the wood blank segments on both sides of the linear compression load, i.e. the input roller, have the tendency to lift off the rigid support. This deformation effect is pressure dependent and also depends on the consistency of the wood blank exposed to the linear pressure effect.
Since wood is a natural material, the ratio of irregularities to the straightness of the underlying surface is very variable. If one works with only one input roller in front of the machining shaft, the wood is pressed together on the feeding table and as soon as it leaves the advance assembly it springs back to its natural shape and therefore is not planed straight.
Attempts have been made to limit this effect by means of the fine adjustment and setting of the input roller. Fine setting means to select low contact pressure values. However, this step has its limitations, since wood blanks have by nature relatively high size variations. Hence even with the fine setting secure holding of the thinnest wood blank under the impact of cutting reactions occurring during the onset of cutting must be ensured.
In the case of extremely thick wood blanks even the finely set contact pressure can assume undesired values. In the case of thick zones in the length of the wood blank and in the case of a variable consistency independent from the thickness, bending effects of various extent still occur, translating into a reverse motion of the pressed-through wood, which again has a major influence on the straightness of the wood.
Because of all that, whenever possible the known processes will avoid the input roller. The expensive manual feeding and the acceptance of correspondingly higher residue and waste quotas leads to a higher straightness of the finished wood pieces and therefore has a more beneficial end effect.
U.S. Pat. No. 2,998,038 refers to the disadvantage of input rollers in wood processing machines. It is mentioned that the blanks which are longitudinally warped are also under a transverse strain when passing under the input roller, so that additional processing defects occur. Consequently, input rollers should be avoided and the wood blanks should be manually pushed in, until the first advance roller downstream of the first rotating tool takes over the conveying operation.
However, in the case of very short wood blanks even manual feed can create problems, so that one is compelled to work with the input roller and to tolerate irregularities.
There is also a so-called "calibrating" of wood blanks, where the rectilinear precision does not count too much, so that in these cases the input roller is always used, mostly because this method is simpler and more convenient than the previously described methods.
For mechanical feeding which replaces the manual feeding, conveyors can be arranged outside the respective processing machine on the input side, provided that these conveyors can be simply and conveniently adjusted to the required advance speed and provided that the wood blanks are long enough, so that these conveyors can act on the wood blanks until their front end reaches the advance roller.
However, in the case of many wood blanks from which, for instance, precision profiles for windows, furniture or the like are supposed to be made, the precondition of sufficient length is not met. Therefore, at least for this kind of work only the solution of manual feeding remains, which is expensive and involves residues and waste.
DE A 145 32 00 discloses a process of the aforementioned kind which avoids manual feeding and input rollers and wherein the wood blanks are pushed forward mechanically only as long as it takes for the transport or advance roller operating downstream of the rotating tool to pick up the conveying work.
Thereby a machine is used which has a chute-like storage the next blank to reach the tool. Long idle periods while the machine is operating are expensive and prevent quick, economic work.
The elimination of the input rollers and the manual feeding slows down the work and the timing, requires long refitting times and leads to an expensive, disturbance-prone machine. No advantage is achieved in comparison to manual feeding, rather the contrary applies. Input rollers are disadvantageous, but when compared with this known process, they are quicker and do not require refitting times, etc.